Nanotechnology is a rapidly developing science that is leading to numerous useful applications in a variety of fields. A primary limitation in further applications of nanotechnology is the ability to efficiently and rapidly produce optimal size and quality of nanoparticles in the desired mediums for use, especially in relation to nanofluids, which are suspensions of nanometer-sized particles in a base fluid. It is very tedious and difficult to manufacture adequate quantities and qualities of nanofluids for the high demand which has developed.
Nanofluids have the potential to impact a myriad of industries and technologies including the area of advanced thermally conductive fluids. For example, when nanoparticles having high thermal conductivities are added to a base fluid the resulting nanofluid often has significantly higher thermal conductivities than that of the base fluid. For example, the heat-transfer capability of ethylene glycol increased by 40% when only 0.3 percent of 10 nanometer size of pure copper nanoparticles were suspended in it. Nanofluids have great potential as heat transfer fluids in many practical applications, such as different thermal systems, electronics, nuclear and biomedical instrumentation and equipment, transportation and industrial cooling, and general thermal management (heating and cooling).
Nanofluids are currently produced by two methods. One method, known as the two-step method, involves first producing the nanoparticles, either pure metals or typically metal oxides, which are then dispersed into the base fluid. In practice this approach has not worked well, particularly for metallic particles, since they tend to oxidize and agglomerate.
The second known method is a one-step preparation process in which nanocrystalline particles (“nanoparticles”) are produced by direct evaporation and deposition onto a low vapor pressure liquid. Nanoparticles produced by this one-step method have tremendous potential. See, U.S. Pat. No. 6,221,275 issued to Choi et al. on Apr. 24, 2001, which is hereby by incorporated by reference in its entirety. However, the current one-step method is a short batch process with limited control over a number of important parameters including those that determine nanoparticle size within the formed nanofluid. Therefore, state of the art systems can only produce nanofluids with nanoparticles equal to or greater than about 10-20 nm. There is a need in the art to produce nanofluids incorporating nanoparticles less than 10 nm.
One parameter that determines the size of the nanoparticles within the nanofluid is the chamber (system) pressure of the system. The chamber pressure is principally determined by the saturation vapor pressure of the base liquid in the vacuum chamber. As the liquid is heated, the chamber vapor pressure rises, and thus, it is critical to keep the liquid temperature low and as constant as possible throughout the process. Without sufficient cooling of the fluid, the temperature will rise mostly due to radiation heat input from the heater, as well as heat input from the impinging nanoparticles and heat gain from the surroundings. An increase in the temperature of the fluid can lead to evaporation of the fluid and other negative results. Therefore, controlling the temperature of the fluid and pressure within the system are very important.
Another important factor in determining the size of the nanoparticles within the nanofluid is the distance the evaporated nanoparticles need to travel before being absorbed by the fluid. Generally, the smaller the distance, the smaller number of gaseous atoms, molecules and particle collisions, and the smaller size of the nanoparticles (all other factors being equal).
For these reasons, it is difficult to make larger quantities of nanofluids or a nanofluid with a large concentration of desired size nanoparticles using current state of the art methods. Also because of the complex nature of the process, it is difficult to control nanoparticle size, the latter being very important.